A composite electrical contact such as a rivet-shaped bimetallic contact which is composed of a metallic contact or head portion and a shank portion made of a metal different from the metal of the contact portion and bonded to a bottom surface of the contact portion, is used by fixing it to a support plate by clinching a free end of the copper or copper alloy shank portion against the support plate. The composite electrical contact such as a bimetallic or trimetallic electrical contact thus caulked to a hole of the support base plate is mounted in an electrical appliance for making various electrical control operations such as opening and closing electric circuits.
The contact or head portion of the composite electrical contact of the kind mentioned above is generally made of a silver alloy, in silver matrices of which particles of metal oxides such as tin oxides are precipitated, so that it can stand up well to a high temperature caused by electric arcs generated about the contact portion when the contact is switched on and off.
The shank portion bonded to a bottom surface of such thermal resistant head portion is made of a metal having a high electric conductivity such as copper and copper alloys, so that electric currents can flow efficiently to and from the head portion. The shank portion can contribute also to dissipate the heat generated at the head portion, and is easy to be caulked to a support plate which is also made of copper or copper alloys.
After the shank portion is passed through a hole provided to the support plate and having a diameter nearly equal to the diameter of the shank portion, its free end is clinched and caulked to the support plate.
This caulking is not so easy if it has to ensure hermetical bonding completely between the shank portion and the support plate. That is, it is nearly impossible to bond them completely airtightly so that there will be not left any gap between the circumference of the shank portion and the hole and between the clinched free end of the shank portion and the support plate.
When the contact caulked to the support plate is operated, its temperature rises, especially at the gap. And, when the shank portion of the contact is subjected to air at an elevated temperature, copper of the shank portion exposed to the gap between it and the support plate becomes oxidized in a short period of time. The electrical conductivity of the shank portion which forms green rust on account of oxidation lowers, and the electrical conductivity and heat dissipation characteristics of the contact as a whole lower consequently, resulting in that the temperature of the contact rises rapidly and the contact will be welded.
In view of the above, this invention is to provide a method of making a novel composite electrical contact, shank portion of which shall be free from a green rust when it is caulked to a support plate and even when subjected to a severe switching operation.